1. Field of the Invention
The present invention relates to a five-speed automatic transmission, and in particular, to an automatic transmission including combination of a four-speed main transmission and an over-drive transmission of a front disposition type for achieving the five speed ranges.
2. Description of the Background Art
A five-speed automatic transmission of a type "W5A030" manufactured by Benz Corporation (Automobilteschnische Zeitschrift 91, 1989 September, pp 445-447) is a five speed automatic transmission including the combination of a four-speed main transmission provided with three sets of planetary gears and an over-drive transmission. This five-speed automatic transmission includes the combination of a simple four-speed main transmission of a Rabinio type, which does not require switching of inputs, and an over-drive transmission of a rear disposition type, and has such good speed change steps that the gear ratio step decreases as the speed range rises.
The five-speed automatic transmission described above, however, has a disadvantage in that the reverse gear ratio is excessively high. In the above example, although the gear ratio of the first speed range is 3.871, the reverse gear ratio is 5.586. The high reverse gear ratio, which is 1.44 times as large as the first speed gear ratio, excessively increases driving force in reverse driving, which may cause a sudden start sensation, slip of tires, excessive engine braking and other problems. Such problems are particularly troublesome in reverse driving wherein smooth driving requires delicate operation of the accelerator.
Further, in the over-drive transmission section, frictionally engaging elements bear a large torque, leading to an increase in weight and difficulty in controllability. For example, a clutch of the over-drive transmission is required, in the forward first to fourth speed ranges, only to transmit an engine brake torque, but in the reverse driving is required to bear a torque which is increased an amount corresponding to the speed reduction for reverse. As a result, the clutch requires many frictional members for ensuring the required torque capacity, which results in an increase in weight and the potential for excessive engine-braking in forward drive. Therefore, the clutch is additionally provided with a centrifugal hydraulic pressure cancelling mechanism for improving controllability.
Although the reverse gear ratio may be reduced to overcome the above-noted disadvantages, this affects the gear ratios for the forward driving operation, and thereby the gear ratios cannot be set at values maintaining good speed-change steps. Also, while the gear ratio might be reduced by operating the over-drive in reverse, in practice this is impossible because the over-drive transmission includes a one-way clutch for reducing shock and the one-way clutch is locked during operation in the opposite direction.
In view of the above-noted problems, it is an object of the present invention to provide a five-speed automatic transmission, which includes the combination of a main transmission provided with simply coupled three sets of planetary gears and an over-drive transmission, and which achieves a good reverse gear ratio while maintaining appropriate forward speed change steps, and also eliminates excessive torque bearing by respective force transmitting parts, so that the whole automatic transmission has a reduced size and weight and thus is improved in versatility for mounting in a vehicle.